Tension anchor system for offshore apparatus



United States Patent [111 3,550,549

[72] Inventor Edward E. Horton [56] References Cited Portuguese Bend,Calif- UNITED STATES PATENTS Q1;- 38*?? 3,466,680 9/1969 Schirtzinger114/230x 45 Patented Dec. 2;, 1970 FOREIGN PATENTS [73] Assignee DeepQil I ec h no l ogy,lnc. 598,037 2/l948 GreatBritain 114/230 Long Beach,Calif.

Primary Examiner-Trygve M. Blix a corporation of CaliforniaAttorney-Miketta, Glenny, Poms and Smith ABSTRACT: A tension type anchorsystem for offshore apparatus for conducting well drilling, productionor the like operations including a pair of upper and lower tensionmeans, the lower tension means being anchored and vertically maintainedby a subsurface buoy at a selected depth. The upper tension means isadapted to move longitudinally relative to the lower tension means andin extended position the upper ten- [54] TENSION ANCHOR SYSTEM FOROFFSHORE APPARATUS 9 Claims, 4 Drawing Figs.

[52] US. Cl 114/230, sion means and the lower tension means are providedwith a l l4/0.5 connecting means for directly transmitting tensionforces in- [5 1] Int. Cl B63b 21/00, dependently of the subsurface buoy.ln nonextended relation, B63b 35/00 the upper tension means is supportedby the subsurface buoy {50] Field of Search 1 14/05, in looselyinterlinked generally parallel relation with the lower 0.5T, 230; 9/80?tension means.

TENSION ANCHOR SYSTEM FOR OFFSHORE APPARATUS BACKGROUND OF THE.INVENTION Offshore well apparatus includes provision of platformstructures supported above a water surface at a selected height asdetermined by various conditions and supported by several differenttypes of supporting constructions. In a type of construction with whichthe present invention is concerned, a working platform is supported by asubsurface buoyant means which is held at a selected location by aplurality of anchor means and tension means, such as cable or chain,connecting the anchor means with the buoyant means. The buoyant means isprovided with a positive buoyancy sufficient to maintain a selectedtension on the tension means and to support the working platform undervarying conditions of load and environmental conditions. Thus, thetension means for a tension leg platform" requires that the platform beheld at a selected location and height, and further when a permanentanchoring system is desired, the tension means should remain in avertical position at a sufficient distance below the height of the waterso that the anchoring system will not present a navigational hazard.

Prior proposed tension means have included cables extending from theanchor at the ocean floor to a subsurface buoy, the subsurface buoybeing then connected by a chain to a surface buoy or platform. In oneprior proposed system, the upper chain portion is passed through ahawsepipe on a pon toon for cooperable engagement with a jack means forimparting tension to the tension anchor means. Such prior proposedsystems are satisfactory, however, it should be noted that the use ofchain in subsea installations presents a difficult problem of preventingrust and corrosion forming on the chain links since they are difficultto completely protect with corrosion resistant coatings, particularly atadjacent loop ends of each chain link. Thus, over a chain of substantiallength there are a multitude of places which, although initiallyprotected by a protective or corrosion resistant coating, a may rapidlybe subjected to wear and penetration of the coating by relative movementbetween chain links to thus expose the chain metal to corrosion. Inaddition, chain of this type has a relatively low strength to weightratio. Use of such chain also requires a relatively large surface buoyto support the weight of the chain. In the event of failure of thesurface buoy and an insufficient design or failure of subsurface buoyswhich support lower tension means, a progressive failure of the entiretension anchor system may occur since the entire anchor tension arraymay sink. Moreover, in such tension systems employing heavy anchor chainsupported by a surface buoy, when water level would vary because of waveor tide conditions, the upper chain portion would slacken as the waterlevel fell and such slack. would accumulate at the lower end of thechain just above the subsurface buoy and would often cause damage to orfouling of the anchor system. Obviously this condition could be avoidedby shortening the upper chain portion and maintaining the chain taut bypulling the surface buoy under water. Such underwater location close tothe surface is not desirable because of the navigational hazardscreated.

The present invention contemplates a tension anchor system whichobviates many of the undesirable characteristics of prior proposedtension anchor means and provided provides a novel anchor tension means.The tension anchor means contemplated by the present invention providesa structurally reliable safe tension anchor system utilizing tensionmembers having a greater strength to weight ratio than provided bychain-type tension members and which does not become a navigationalhazard when not is use. The present invention also provides an anchorconstruction which may be effectively protectively coated withcorrosion-resistant materials throughout its entire length and thusbecomes lee less susceptible to corrosion. Moreover, the tension anchorsystem contemplates an arrangement of tension members wherein forcetransmitting areas are reduced to a minimum.

LII

The primary object of the present invention is to design and provide atension anchoring system of novel construction for maintaining aplatform of tension leg-type at a selected site and for maintaining asubsurface buoyant or pontoon means supporting such a platform at aselected depth in the water.

An object of the present invention is to disclose and provide a tensionanchoring system wherein the entire tension means is maintained invertical relation beneath the ocean surface and without employment of asurface buoy.

Another object of the present invention is to disclose and provide atension anchoring system readily adapted to be protectively coated andwherein the tension anchor system may be utilized with minimum or nodamage to the protective coating.

A further object of the invention is to disclose and provide a tensionanchoring system wherein a pair of tension members are employed and areinterlinked together so that they may be maintained in a verticalposition beneath the ocean surface and without reliance upon a surfacebuoy.

A still further object of the invention is to disclose and provide atension anchoring system wherein upper and lower tension members aresupported in collapsed or nonextended rela tion by a subsurface buoywhich maintains at least one of said tension members under a selectedtension force and which is adapted and capable of supporting the deadweight of the upper or there other tension member.

A still further object of the invention is to disclose and provide atension anchoring system wherein upper and lower tension membersinterrelated as described above when in nonextended position, areinterrelated by a connecting means for directly transmittinglongitudinal tensional forces from one member to the other withouttransmitting such forces through a subsurface buoy which normallysupports and maintains tension on the lower tension member.

Various other objects and advantages of the present invention will bereadily apparent from the following description of the drawings in whichan exam exemplary embodiment of the present invention is h shown.

In the drawings:

FIG. 1 is a side elevational view of a tension means embodying thisinvention in nonextended relation.

FIG. 2 is a side elevational view showing the tension means of FIG. l inextended relation and connected to a floating submersible pontoon meansprior to submerging.

FIG. 3 is an enlarged fragmentary side elevational view of tensionmember connecting means at the submersible buoy means.

FIG. 4 is a vertical sectional view taken in the plane indicated by linelV-lV of FIG. 3.

In FIG. I there is shown a tension means generally indicated at It) foranchoring offshore well apparatus of tension leg type wherein asubmersible buoy support means is anchored in water over a selectedlocation and at a selected depth for supporting a working platform at aselected height above the water surface. Tension means 10 generallycomprises a pair of tension members 11 and I2 associated with asubmersible or subsurface buoy means 14 which is located below thesurface 15 of the water a selected depth which is sufficient to preventbuoy 14 from becoming a navigational hazard. A marker buoy 16 floats onthe water surface 15 to indicate location of the anchor site and may beattached to tension member 12 by a light pennant line 17. Tension meansmay be anchored at selected location by an anchor means 18.

Anchor means 18 bay comprise any suitable anchor body 19, such as adeadweight mass of concrete or other heavy dense anchor material, or itmay comprise a type of c anchor means which is driven or drilled intothe ocean floor, such as a pile or the like. Anchor body 19 may beprovided with an eye 20 which provides a connecting means for the lowerend of tension member 11.

The pair of tension members 11 and 12 may be similar in constructioneach including a pair of tension cables 21,21 and 22, 22 respectively.The lower ends of cables 21 are con nect'ed and joined by a suitablemetal bail 23 which passes through and engages anchor eye 20. The upperends of cables 21 are connected to an upper bail 24. Bail 24 is providedwith an upstanding plate 25 having a port 26 at each upper plate cornerfor receiving chain link 27 connected to a parted lug 28 secured as bywelding at 29 to wall 30 of one of the buoy members 31 forming buoymeans 14. Pivotal connections provided by ported lug 28 and link 27,plate 25 and link 27, and pivotal connections provided between ends ofcable 2ll and the bails 23 and 24 distributes the load between tensioncables 21. Tension member 11 is thus pivotally supported and hung frombuoy l4.

Tension member 12 similarly includes tension cables 22, the lower endsof which are pivotally connected at 33 to a bail 34. The upper ends oftension cables 22 are similarly pivotally connected at 35 to a bail 36.It should be noted that the lower bail 34 lies between cables 21 andthat the continuous loop formed by the lower tension cables 21 and bails23, 24 is linked and interconnected with the loop formed by the uppercables 22 and bails 34, 36. Tension members ill and l2 are thus arrangedalong adjacent generally vertical parallel lines, tension member 12being longitudinally telescoped or collapsed for a substantial part ofits length with respect to tension member 11 (FIG. 1) and when extended(FIG. 2) tension member 12 lies along a line which is generally acontinuation of the vertical direction or line of lower tension memberill.

The submersible buoy means 14, in this example, comprises a pair ofparallel horizontally spaced apart elongated buoy members 3i. ofcylindrical cross-sectional configuration. Buoy members 31 are held inspaced apart relation to define an opening 44 by a plurality oflongitudinally spaced top transverse bars 40 secured in suitable manneras by welding to the wall of buoy members 31. A plurality of bottomtransverse bars 41 may also interconnect buoy members 31 below ahorizontal diametrical plane passing through the buoy members 31. Bars41 may be suitably secured as by welding to buoy member 311.

As best shown in FIG. 1 when the tension members I! and 12 are innonextended relation and alongside each other bail 36 of upper tensionmember 12 is supported and carried by one of bars 40 of buoy 14, thespaced tension cables 22 as shown in FIG. 4 passing on opposite sides ofa set of vertically aligned bars 40 and 41. Thus, in nonextendedrelation buoy M supports and carries tension member 12 and alsomaintains a selected tension on tension member 11 so that tension memberill will remain substantially vertical.

in fully extended relationship of tension members 11 and 12, the bottombail 34 of tension member 12 engages at 42 top bail 24 of tension memberill. Opening 44 provided between buoy members 3i permits free passage ofcables 22 therethrough so that tension forces or leads imposed upontension member 12 by a suitable tension applying means 45 carried by aworking platform structure generally indicated at 46 may be transmitteddirectly between tension members Ill and 12 at 42 and fullyindependently of the buoy 14.

A working platform structure is only partially illustrated in FIG. 2 andgenerally comprises a submersible buoyant support means 47 carryingplatform legs 48 for supporting thereabove a platform (not shown). Asdescribed in my copending application Ser. No. 735,320, the submersiblebuoyant means 47 may be submerged to a selected depth for supporting theplatform a selected distance above the water surface and may then beimparted sufficient positive buoyancy so as to maintain a predeterminedtension force on tension means 10.

When the tension anchor system described above is employed, it will beapparent from a consideration of FIG. 1 that the entire tension means,including upper and lower tension members 12and ll, is supported invertical relation by subsurface buoy 114 located a selected distancebelow the surface of the water so that it will not constitute anavigational hazard and so that it will be a sufficient distance belowthe surface of the water to permit the positioning of a subsurfacesupport buoyant means such as 4'7 thereabove for anchoring an offshoreplatform structure. In such nonextended or collapsed position, thetension anchor means may be marked by the pennant floating buoy 16 whichis of very light weight and pennant line 17 carries virtually no load.In such collapsed relation, it will be apparent that the only steady orcontinuous contact between the upper tension members 12. and subsurfacebuoy M will occur at the contact area between the inner surface of bail36 and bar 40. Thus, a very minimum of contact area which may producewear and corrosion is provided.

Also in extended relation of the upper and lower tension members it willbe readily apparent that the only steady and continuous contact undertension conditions will be at 42 between top bail 24- of lower tensionmember 11 and bottom bail 34 of upper tension member 12. As above aminimal contact area is provided.

The interlinking of tension loops of cable with minimal contact of thecable under load and no-load conditions provides a construction which acan be readily uniformly coated with a suitable protective compositionand in which the coating is subjected to force loads at only the minimalcontact areas described above.

The interlinking of the loops of tension members 11 and 12 by apivotally mounted or hung top bail 24 on subsurface buoy l4 and freepassage of member 22 through an opening in buoy l4 eliminatestransmission of tension forces through buoy 14 to upper tension memberl2 and longitudinal or axial loads are directly transmitted between theupper and lower tension members. Thus. subsurface buoy 14 serves toprovide a sufficient upward tension force on lower tension means 11 tomaintain the lower tension means in vertical relation under virtuallyall conditions and also serves to support the deadweight of the uppertension means when tension forces are not applied to the system and theanchor system is not in use. A permanent anchor system is thus provided;the system being readily available for use while not presenting a hazardto navigation.

It will be readily apparent to those skilled in the art that variousmodifications and changes may be made in the tension anchor systemdescribed above which come within the spirit of the invention and allsuch changes and modifications coming within the scope of the appendedclaims are embraced thereby.

Iclaim:

ll. Tension means for c anchoring offshore well apparatus comprising:

a pair of longitudinally relatively movable tension members arrangedalong adjacent generally vertical parallel lines; one tension memberhaving its lower end portion adapted to be anchored;

submersible buoy means for holding said one tension member underselected tension at a selected depth and for supporting the othertension member alongside said one tension member in depending relationto said buoy means when said tension members are in nonextendedrelation; and

means at said buoy means for direct connection of said tension memberswhen said tension members are in extended relation to transmit tensionforces independently of said buoy means.

2. Tension means for anchoring offshore well apparatus comprising:

a pair of longitudinally relatively movable tension members arrangedalong adjacent generally vertical parallel lines; each tension memberbeing in the form of a continuous vertical loop;

one tension member having its lower end portion adapted to be anchored;

submersible buoy means for holding said one tension member underselected tension at a selected depth and of for supporting the othertension member when said tension members are in nonextended relation;and

means at said buoy means for direct connection of said tension memberswhen said tension members are in extended relation to transmit tensionforces independently of said buoy means.

3. A tension means as stated in claim 2 where in said loops of eachtension members are loosely interlinked.

4. A tension means as stated in claim 1 wherein said submersible buoymeans includes an opening for vertical passage of said other tensionmember.

5. A tension means as stated in claim 4 wherein said submersible buoymeans includes means at said opening for supporting engagement withmeans at the upper end portion of said other tension member.

6. A tension means as stated in claim 1 wherein said submersible buoymeans includes spaced buoyant members defining opening for passage ofsaid other tension member.

7. A tension means as stated in claim 1 wherein each tension memberincludes a pair of spaced cables, and bail members connecting adjacentends of said cables.

8. In an anchor system for use on offshore operations, the combinationof:

anchored vertically extending tension means including a force connectingmeans at the upper end thereof; longitudinally relatively movabletension means extending alongside and loosely interengaged with saidanchored tension means;

one end of said movable tension means being supported by said upper endof said anchored tension means and adapted to be attached to a pennantline leading to a surface buoyant means under nonforce transmittingconditions;

the other end of said move movable tension means being engageable withsaid connecting means for directly transmitting tension forces betweensaid anchored and said tension means under force transmittingconditions.

9. In an anchor system as stated in claim 8. wherein said anchored meansincludes:

a subsurface buoy at the upper end of said anchored vertically extendingtension means; and

said subsurface buoy imparting tension to said anchored tension means insufficient amount to additionally support in vertical relation saidmovable tension means under nonforce transmitting condition whereby saidanchor system is maintained in vertical position at a selected depthbelow the surface of the water.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,3,550,549 Dated December 29, 1970 Edward E. Horton Inventor(s) It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

2 line 27, cancel "there"; line 38 cancel "exam"; line 39 cancel "h";line 66, cancel "bay" and insert may line cancel "c". Column 3, line 5,"parted" should read ported line 52, "leads" should read loads Column 4,line 45, cancel "c". Column 5, line 5, "1" should read 2 line 1 18, "on"should read in Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,J-R. WILLIAM E. SCHUYLER, Attesting OfficerCommissioner of Paten USCOMM- DC 603 Column 1 line 39 cancel "a"; line72, cancel "lee". C01

"1" should read Z line 15, "1" should read 2 1i:

